1. Field of the Invention
The present invention relates to a focus detecting system for a camera, and more particularly to a TTL type focus system which can detect a focusing state of an imaging lens by light fluxes from the imaging lens.
2. Description of the Prior Art
A focusing detecting system which detects a focusing state of an object lens by detecting a relative positional relation of a plurality of object images formed based on light fluxes from the object lens, by photo-sensing means has been known, for example, by U.S. Pat. No. 3,875,401 issued on Apr. 1, 1975.
When such a focus detecting system is used in a single lens reflex camera which allows exchange of lenses, the following problem arises. In such a focus detecting system, it is necessary to form a plurality of object images based on a light flux which passes through a peripheral area of the object lens in order to enhance a detection precision. Thus, when the detector is used in the single lens reflex camera, if the path through which the light flux for forming the plurality of object images is set in a lighter one (having a smaller aperture F number) of the object lenses mountable to the camera body, a portion of the light flux for forming the object images is shielded when a darker lens (having a larger aperture F number) is mounted to the camera body and the focusing state cannot be exactly detected.
In order to resolve the above problem, a focus detecting system is proposed in Japanese patent application Laid-Open No. 118019/1980 (laid open on Sept. 10, 1980), in which four secondary focusing lenses are arranged behind an image plane of an object lens to form a plurality of object images by a light flux passing through a center area of the object lens around an optical axis and form a plurality of object images by a light flux passing through a peripheral area of the object lens, and outputs of four photosensing means for detecting the object images formed by the secondary focusing lenses are selected in accordance with the object lens mounted on the camera body to switch a combination of the object images for detecting a relative position in accordance with the object lens. In this detector, the relative positional relation of the object images by the light flux passed through the center area around the optical axis of the object lens and the relative positioned relation of the object images by the light flux passed through the peripheral area of the object lens can be selectively detected as required independently from the type of the object lens mounted on the camera. Accordingly, the above problem is resolved.
A focus detecting system which forms four object image without arranging four secondary focusing lenses is disclosed in Japanese patent application No. 150520/1982 filed on Aug. 30, 1982 by the applicant of the present application. Since this system does not require the four secondary focusing lens, a space is saved. FIG. 1 shows a perspective view of a focusing state detector shown in the above application. In FIG. 1, a first object image formed on an anticipated imaging plane S of an object lens 1 by light fluxes L.sub.1 -L.sub.4 (light fluxes L.sub.1 and L.sub.2 pass through a peripheral area of the object lens 1, and light fluxes L.sub.3 and L.sub.4 pass through a center area of the object lens 1 around an optical axis L) is reformed, as second object images, onto photo-sensing planes of photo-sensing means 9a, 9b, 9c and 9d, which are photo-electric conversion elements arranged on a secondary imaging plane 7, through a field lens 3, light dividing means 5 and a secondary focusing lens 6.
A relative positional relation of the second object images is selectively detected by the photosensing means 9a-9d to determine a focusing state of the object lens 1. A light shielding plate 2 having a slit 2a is arranged near the anticipated imaging plane S, and the light split means 5 and the secondary focusing lens 6 are appropriately arranged such that four second object images by the slit 2a do not overlap to each other on the secondary imaging plane 7.
The light dividing means 5 is a wedge-shaped prism having two light deflectros 5a and 5b. The light deflector 5b deflect the light fluxes L.sub.1 and L.sub.2 impinged thereto to the opposite sides with respect to the optical axis L of the object lens 1 by an equal angle. The other light deflector 5a deflects the light fluxes L.sub.3 and L.sub.4 impinged thereto underword with respect to the optical axis X. It is arranged in front of the light deflector 5b.
Thus, the object image formed on the anticipated imaging plane S is reformed, as four second object images, on the secondary imaging plane 7 through the light split means 5 and the secondary focusing lens 6. Numeral 4 denotes a mask plate having apertures 4a and 4b for limiting the light fluxes impinged to the light split means 5. The light fluxes are split by the apertures 4a and 4b symmetrically around the optical axis L. the field lens 3 is arranged near the anticipated imaging plane S of the object lens 1 and it focuses the light dividing means 5 at the vicinity of an exit pupil of the object lens 1. The field lens 3 has such an index of refraction that the whole light dividing means 5 is imaged within an exit pupil aperture of the object lens 1.
The second object images formed on the photosensing means 9a and 9c are formed by the light fluxes L.sub.1 and L.sub.3 transmitted through one side of the exit pupil of the object lens 1, and the images formed on the photo-sensing means 9b and 9d are formed by the light fluxes L.sub.2 and L.sub.4 transmitted through the other side of the exit pupil. They are formed independently from each other. Thus, when the object lens 1 is a light lens, the output signals from the photo-sensing means 9a and 9b or all of the photo-sensing means 9a-9d are used, and when the object lens 1 is a dark lens, the output signals from the photo-sensing means 9c and 9d are used to detect the relative positional relation of the object images.
In such a system disclosed, for example, in Japanese patent application Laid-Open No. 118019/1980, when the focusing state of the object lens is determined by the relative positional relation of the plurality of object images formed by the light fluxes transmitted through the peripheral area of the object lens, the object images formed by the light fluxes transmitted through the center area of the object lens are neglected. As a result, the light fluxes which enables the discrimination of the focusing state of the object lens are not fully utilized. In such a detector, the precision of detection of the focusing state is low when an object is dark.